Organic fluids for heat pipes

ABSTRACT

Certain aromatic and aliphatic organic compounds are disclosed as working fluids for heat pipes, thus affording improved operation in the temperature range from about -40* C. to about 500* C. These fluids have critical temperatures in excess of 300* C., kinematic viscosities less than 500 centistrokes at 0* C., and the numerical product of surface tension and latent heat of vaporization is greater than 2500 dyne-calories per gramcentimeter.

. United States Patent [72] Inventor Kenneth L. Mcflugh Kirkwood, Mo. [21] Appl. No. 667,970 [22] Filed Sept. 15. 1967 [45] Patented July 27, 1971 [73) Assignee Monsanto Company Saint Louis, Mo.

{54] ORGANIC FLUIDS FOR HEAT PIPES 2 Claims, 1 Drawing Fig.

[52) U.S. CL 165/1, 165/105, 252/73, 252/77, 252/79 [51] Int. Cl 1-28d 15/00 [50] Field 01 Search 165/105, 1; 62/1 19; 244/1; 252/73, 77 79 [56] References Cited UNITED STATES PATENTS 1,120,246 12/1914 Schmidt 165/105 2,883,591 4/1959 Camp 317/234 2,886,746 5/1959 Saby 165/105 X 3,085,180 4/1963 Zwijsen.... 3,229,759 1/1966 Grover OTHER REFERENCES Deverall, J. E. et al. Devices LA- 321 1, 4/1965 pp. 1,8,11

Feldman, Jr. K. T. et al., Heat Pipe Mech Engineering, 2/1967 pp. 30, TH. A72

Grover, G. M. et al. Structures Journal of Applied Physics, 1964 pp. 1990, 1991 0C1. J82

Sandia, Heat Pipe Conf, SC- M- 66- 623, 10/1966, pp. 46, 83

Primary Examiner-Albert W. Davis, Jr. Attorneys-Neal E. Willis, J. E. Maurer and William H. Duffey ORGANIC FLUIDS FOR HEAT PIPES The heat pipe, sometimes called a vapor chamber, is a simple and lightweight heat transfer device having very high thermal conductivity. In its simplest form, the heat pipe comprises a narrow, sealed tube which houses a porous wick and a working fluid. Sufficient working fluid is put into the heat pipe to wet the entire wick. The wick is held tightly and uniformly against the inside wall of the tube, forming a capillary structure capable of efficiently moving the working fluid.

Functioning ofthe heat pipe is explained by two well-known physical principles, viz, vapor heat transfer and capillary attraction. The fluid is evaporated by the addition of heat at one end of the tube, called the heat input zone, and is condensed by the removal of heat at the other end, called the heat removal zone. These zones are generally interchangeable. Upon vaporization of the fluid, the pressure in the heat input zone is elevated, thus causing the vapor to move toward the heat removal zone. Following removal of heat from the heat pipe, the vapor condenses and is reabsorbed in the wick. With the depletion of fluid in the heat input zone and additional concentration of fluid in the heat removal zone, a pressure differential within the capillaries of the wick produces a flow which returns the fluid to the heat input zone.

The nature of the working fluid has a profound effect on heat pipe performance. Besides permitting continuous and automatic operation of the heat pipe, the fluid must be efficient in the transfer of heat within the pipe. Properties of the working fluid which are influential on heat pipe performance include thermal stability, latent heat of vaporization, surface tension, viscosity, density, and wettability of the heat transfer surface;

The literature discloses various working fluids for heat pipe applications. Among these are water, ammonia, glycerine, and molten salts. Also included are low melting point metals such as cesium, potassium, sodium, lithium, lead, bismuth and indi- At operating temperatures below 350 C., working fluids such as water and ammonia have been used to advantage. They provide a combination of high latent heat, good surface tension, and low viscosity. Water is deficient, however, in those heat pipes where temperatures below C. are experienced because water expands upon freezing. Ammonia, on the other hand, is usable only at temperatures below 0 C.

Glycerine, with a critical temperature in excess of 500 C., permits higher temperature heat pipe operation than does water, and the latent heat of vaporization of glycerine is four times that of water. The suitability of glycerine as a heat pipe fluid, however, is markedly diminished because of its high viscosity. The viscosity of glycerine at C., for example, is

about 1,000 times that of water. While the kinematic viscosity of water at 20 C. is L000 cs., that of glycerine is 1069 cs. Such unduly high viscosity impedes the flow of fluid through the capillary member of the heat pipe, thus preventing instantaneous startup, and, in many instances, rendering the device inoperative.

The low melting point metals enumerated above, when in their liquid form, possess high latent heat of vaporization, high surface tension, and relatively low viscosity. The main deficiency of these materials for heat pipe use, however, is their high melting and boiling points, which restrict operation to a minimum temperature of approximately 350 C. for even the lowest boiling member of the group, viz, cesium. From this minimum temperature up to approximately l,600 C., the liquid metals can be employed as heat pipe fluids. At the high temperatures, however, the liquid metals tend to form nonmetallic compounds between themselves and the refractory elements of the heat pipe, thus necessitating exotic materials of construction. A further consideration is the solubility of the liquid metal in the refractory materials of the wick and the tube. Lithium and potassium, moreover, have presented heat transfer difficulties because of poor to marginal wetting properties on a metal surface. Because the alkali metals, e.g. cesium, lithium and potassium are extremely reactive, handling problems are encountered with their use. Cesium, furthermore, is not in plentiful supply.

It has now been discovered that certain organic compounds are ideally suited as working fluids for heat pipes where the operating temperature range is from approximately ---40 C. to 500 C. The compounds taught by the present invention will be shown to be markedly superior to known heat pipe fluids for use in the operating range stated above. It will be further shown herein that, for reasons not fully understood, only certain types of organic compounds possess the superior properties required for efficient heat pipe performance. Some organic compounds closely related to those taught herein, for unexplained reasons, are wholly unsuited for the instant use. it will further be shown that the organic compounds taught herein are noncorrosive to conventional materials of construction.

It is an object of the present invention, therefore, to improve the operating range of heat pipes through use of certain organic working fluids. A further object of the present invention is to disclose specific classes of organic compounds which provide efficient heat pipe performance, particularly in the range from 0 C. to 350 C. where liquid metals are generally inoperative. Still another object of the present invention is to define those fluid properties which organic fluids must possess to achieve superior heat pipe performance. Yet another object of the present invention is to disclose heat pipe fluids which do not attack, corrode or react with conventional materials of construction during high temperature operation. Other aspects, objects, and advantages of this invention will become apparent from a consideration of the accompanying disclosure and drawing, and the appended claims.

In the drawing:

The sole FIGURE is a sectioned view of a typical heat pipe configuration.

Broadly stated, the present invention provides substantial improvements in the operating range of heat pipes through the use of certain organic compounds as working fluids. The organic fluids taught by the present invention have critical temperatures in excess of about 300 C. and the numerical product of the latent heat of vaporization (at normal boiling point) and surface tension (at 20 C.) of each fluid is at least about 2,500 dyne-calories per gram-centimeter. The kinematic viscosity of each fluid is less than about 500 cs. at 0 C. These fluids offer remarkable advantages for heat pipes operating in the range from about -40" C., to about 500 C.

Typical of the compounds taught by the present invention are aromatics such as aniline, pyridine, thiophene, and substituted derivatives thereof, where the substituents may be, for example, alkyl, aryl, halogen, nitrile, nitro, hydroxy, amino, alkoxyl, phenoxy, mercapto, phenyl, or multiples or combinations of these substituents.

Further included within the scope of the present invention are compounds having condensed heterocyclic rings such as benzofuran and quinoline. Also included are heterocyclic compounds containing more than one hetero atom such as diazole, triazole, dioxane and pyrimidine. Heterocyclic compounds containing combinations of hetero atoms of oxygen, nitrogen and sulfur, for example, oxazole, isothiazole and oxatriazole, are also within the present scope.

In addition to the compounds described above, selectively substituted aliphatic compounds have been found to be outstanding fluids for the practice of the present invention. Exemplary fluids within this class are aliphatic nitro compounds such as nitromethane and aliphatic nitriles such as acetonitrile and propionitrile. Substituted amides such as dimethylformamide, dimethylamide, together with substituted sulfates such as dimethylsulfate, are also within the scope of the present invention.

Referring now to the sole FIGURE of the drawing, a typical heat pipe is illustrated. The housing or tube, denoted by reference numeral 10, is a sealed member or rigid material. The material chosen for housing 10 is a function of the operating environment of the heat pipe and the nature of the work ing fluid. Because the organic fluids of the present invention are compatible with stainless steel, mild steel. and the like. there is no requirement for special materials of construction as been found undesirable for heat pipe applications because of its poor wettability Cesium, therefore, with a boiling point of 670 C. is the next of the group to be considered but it could not be expected to function below approximately 350 C.

is found with certain prior art working fluids. Other materials 5 Since water and glycerine are unsuitable in the range and for housing 10 include glass, ceramic. copper, ni kel, ram liquid metals are limited to a minimum of about 350 C.. the qalu titanium a d lybd d i bi ll range from 0 C. to 350 C. has heretofore been excluded for with ontin ed refere to h d wi h i 10 can b efiicient utilization of the heat pipe because of the lack of outde igned with a wide va ie f et i l configurations, standing working fluids. The organic fluids of the present in- Besides the illustrated cylindrical configuration, housing 10 10 vention answer this needmay be, for example, a disc, a rectangular plate, or can have The '8 Working fluids taught by the Present Invention two or more diameters, may l b uor d bl embrace a large number of chemical compounds. For illustrawalled, i.e., it may receive heat on the inside of a wall and re- PUYPOSeSI the P Properties of Several representajec heat on the id f tive compounds are presented in Table 11 below.

TABLE II Properties of Organic Compounds Melting Boiling Latent Surface Criticla point point Density heat 1 tension temp. 0.) C.) (g./rnl.) (caL/g.) (dynes/cm.) C.)

Pyridine 42 115 0. 98 107. 4 38. 0 346 Aniline -6. 3 184 l. 02 99. 5 45. 5 427 Quinoline 15. 6 237 1. 10 87 45. 0 498 Ch1orobenzene 46 132 1. 11 74. 4 34. 8 350 Thiophenol. 14. 6 169 1. 08 80. 2 40. 5 420 Nitrobenzene 5. 7 211 1. 84. 3 46. 4 483 Benzonitrile 12. 9 191 l. 00 87. 7 30. 1 426 Nitromethane -29 101 1.13 135.0 39. 8 309 Dimethy1forrnamide. 60 150 0.95 125. 4 36. 8 368 o-Chlorophenol 9. 0 175 1.26 74. 5 41. 2 400 Diphenyl st11fide -40. 0 296 1. 2O 68. 4 41. 0 540 Dimethyl sulfate. 26. 8 188 1. 100 40. 1 315 Thiophene 183 1. 06 89. 4 33. 9 307 I Latent heat at normal point.

Again referring to the drawing, the heat pipe wick 11, which is disposed uniformly against the inside wall of housing 10, comprises the capillary structure of this device. Typical materials for wick 11 are fibrous paper, woven cloth, fiberglass, sintered copper, woven stainless steel mesh, and woven molybdenum mesh. Prior to closure of housing 10 during assembly ofthe heat pipe, the working fluid is added in sufticient quantity to saturate wick ll. Thereupon, the housing is evacuated and sealed.

The heat pipe is essentially an isothermal device and, by its nature, is intolerant of any temperature differential along its surface. it will remove energy by evaporation from any point where heat is available, and will deliver energy by condensation wherever a thermal drain exists. Heat may be transferred to or from the heat pipe by radiation, convection or conduction. One specific application of the heat pipe is in space satellites to transfer heat from a radiosotope heat source to a thermoelectric or thermionic electric power generator. On the heat sink side of the power generator, the heat pipe can be used to transfer heat to a radiator or to warm other satellite components. Other obvious uses for the heat pipe are the following: heat transfer from a burner to an oven; heat removal from electric motors, electron tubes, transistors, and the like.

To illustrate the operating temperature range limitations of heat pipes incorporating low melting point metals, Table 1 below presents the pertinent physical properties of seven metals which have been actually used, or considered for use, in prior art heat pipes. Of primary significance in Table l are the boiling points and melting points of the metals enumerated therein.

TABLE I Properties of Metals Melting Boiling Latent Surface point point Density heat tension Mercury -39 356 13. 5 71 476 Cesium" 26 670 1.84 146 60 Potassium." 62 760 0. S3 495 400 Sodium... 98 S80 0. '33 1.005 205 Lithium. 186 1 .336 0. 53 4 ,690 398 Lead 327 1,620 10. 3 205 470 SilVBL. 960 1 ,950 i 4 5 800 2 Surface tension at 20 C.

in II above it is seen that the boiling points of these organic working fluids are sufficiently low and the critical temperatures are sufficiently high to afford heat pipe operat on in the sought-for range from 0 C. to 350 C. Diphenyl sulfide, for example, with a critical temperature of 549 C., will permit operation at temperatures much higher than 350 C.

With further reference to Table [1, it can be observed that the critical temperature of each fluid therein is greater than 300 C., and the product of latent heat and surface tension is at least 2,500 dyne-cal. per gm.-cm.

To insure operability of the heat pipe, it is important that the kinematic viscosity of the working fluid be less than about 500 centistokes at 0 C. All the fluids of Table ll have kinematic viscosities less than cs. at 0 C., and many are substantially less than 100 cs. Pyridine, for example, has a viscosity of approximately 1 cs. at 0 C. Capillary action in the heat pipe is therefore assured with the instant fluids at low temperatures.

Organic fluids within the classes taught by the present invention and which meet the aforementioned requirements of critical temperature, latent heat, surface tension and viscosity are advantageous as heat pipe working fluids.

Organic fluids of aromatic structure constitute the preferred class in the present invention. Certain aliphatic compounds, however, have been found to be suitable for the heat pipe use intended herein. Nitromethane, for example, an aliphatic nitro compound, is within the scope of this invention as evidenced by its properties cited in Table II. The latent heat of vaporization of nitromethane is 135.0 cal./gm. and its surface tension is 39.8 dynes/cm., the product of these two values being 5,370 dyne-cal./gm.-cm., well in excess of the 2,500 minimum figure established herein.

To illustrate an aliphatic compound which is outside the scope ofthe present invention, chloromethane serves as an example. The latent heat of vaporization of chloromethane is I00 cal/gm. but its surface tension is only 15.3 dynes/cm., the product of these two values being 1,530 dyne-cal./gm.-cm. Referring again to Table II, it will be observed that a latent heat value of 100 cal/gm. is typical for fluids of this invention. The surface tension of 15.3 dynes/cm. exhibited by chloromethane, however, is less than one-half of the surface tension of the fluids of Table ll. its critical temperature of 143 C. is also less than one-half of the established minimum of 300 C.

In most instances, halogenated aliphatic compounds such as the chloroalkanes, fluoroalkanes, chloroalkyls, fluoroalkyls, mixed chlorofluoroalkyls, and the like, are undesirable for use as heat pipe working fluids primarily because of their relatively low critical temperatures.

Critical temperature is a vital parameter in the fluids of the present invention and the importance thereof will hereinafter be shown.' Halogenated aliphatic compounds, therefore, are not among the superior organic working fluids of the present invention because their thermal stability is usually limited to the order of 150 C. to 200 C., because of lack of suitable overall fluid range, and because of deficiencies in the produce of surface tension and latent heat of vaporization.

The importance of critical temperature can be explained as follows. If the operating temperature of the heat pipe is allowed to closely approach the critical temperature of the working fluid, there occurs a loss of distinction in physical properties between the vapor and liquid phases. The latent heat of vaporization approaches zero. When this occurs, the heat pipe flow cycle is aborted and the device becomes essentially inoperative. Heat transfer thereupon takes place only by conduction and natural convection in the fluid, and the rates are greatly diminished as compared to those obtained during subcritical operation.

Heat pipe operation may be impeded in another manner which is not attributable to critical temperature per se. That is, if the initial charge of working fluid is inadequate, it is possible for the heat pipe to go dry at elevated temperatures below the critical temperature.

It has been found that selected mixtures or blends of two or more fluids of the present invention can produce certain improved characteristics for heat pipe use. A mixture of phenol and aniline, for example, because of the weak acid-base reaction, forms a liquid salt combination in solution which provides additional heat transfer capacity due to the energy required to dissociate the salt prior to vaporization, this heat to be released upon recombination in the heat removal cycle.

stituents in the compounds of the present invention are alkyl radicals, e.g., methyl, ethyl, propyl, butyl, octyl; aryl radicals,

Within this invention, therefore, are those organic mixtures which form saltlike material or covalent complexes involving shared electrons. It is only necessary that the combined molecule have a liquid range compatible with the operating environment of the heat pipe, and, of course, that the critical temperature, latent heat, and surface tension standards be achieved. Acidic compounds such as phenol, cresol, substituted phenol, thiophenol and carboxylic acids, together with basic compounds such as aniline, substituted aniline, amides, and heterocyclic amine compounds such as pyridine, piperazine, pyrrole, and the like, are within the instant scope.

Illustrative of the various radicals which may appear as sube.g., phenyl, naphthyl, tolyl, xylyl, etc.; aralkyl radicals, erg, benzyl, phenylethyl, etc.; alkenyl radicals, e.g., vinyl, allyl, etc., halogens, e.g., chlorine, fluorine, iodine and bromine; alkoxy radicals, e.g., methoxy, ethoxy, propoxy, butoxy, etc.; acyloxy radicals, e.g., acetoxy,,butoxy, etc.; aryloxy radicals, e.g., phenoxy, naphthoxy, aryloxy-substituted phenoxy, alkoxy-substituted phenoxy, alkyl-substituted phenoxy, aryl-substituted phenxoy, halogen-substituted phenoxy, etc.

All of the heat pipe working fluids taught by the present invention are known organic compounds. Preparations of these compounds are disclosed in the literature, thus permitting practice of the invention by one skilled in the art.

Values for surface tension, expressed in dynes per centimeter, and latent heat of vaporization, expressed in calories per gram and kinematic viscosity, expressed in centistokes, have accepted scientific significance as evidenced by their repeated use in scientific handbooks. It is recognized, of course, that these parameters may be expressed in other units of measurement having different numerical values without altering the actual limits imposed by this invention. Although some variations in measured surface tension occur with changes in fluid temperature, for purposes of the present disclosure it is assumed that the fluid is at a temperature of 20 C. Similarly,

latent heats are quoted at the normal boiling point. Because these parameters have longstanding meaning in the physical arts, the methods of measurement are deemed to be standardized.

While this invention has been described with respect to certain specific examples, it is not so limited, and it is to be understood that variations and modifications thereof may be made without departing from the spirit of the following claims.

The embodiments of this invention in which an exclusive property or privilege I claim are defined as follows:

1. A method of transferring heat which comprises the steps of:

a. vaporizing within a closed chamber an organic fluid selected from the group consisting of pyridine, thiophene, quinoline, chlorobenzene and diphenyl sulfide,

b. transferring heat through the wall of said chamber,

c. liquefying the vaporized fluid through condensation, and

d. absorbing the condensate in a capillary member within said chamber.

2. A heat pipe comprising a closed chamber, a capillary member within said chamber, and an organic working fluid within said capillary member, said fluid selected from the group consisting of pyridine, thiophene, quinoline, chlorobenzene and diphenyl sulfide, 

2. A heat pipe comprising a closed chamber, a capillary member within said chamber, and an organic working fluid within said capillary member, said fluid selected from the group consisting of pyridine, thiophene, quinoline, chlorobenzene and diphenyl sulfide, 